bugfix KubectlGetSingle.execute() error

Fix the problem of getGenericApi failure when the `KubectlGetSingle.execute()` method is executed for the first time

Author: lin1005q

extended/src/main/java/io/kubernetes/client/extended/kubectl/KubectlGet.java

@@ -97,6 +97,8 @@ private boolean isNamespaced() {
 
     @Override
     public ApiType execute() throws KubectlException {
+      refreshDiscovery();
+
       GenericKubernetesApi<ApiType, ? extends KubernetesListObject> api =
           getGenericApi(KubectlGetSingle.this.apiTypeClass);
       try {